The induction technology can advantageously be used at sealing of thermoplastic coated packaging material, including at least one layer of metal foil, preferably aluminium foil. Hereby an inductor as described above is used, which in the metal foil creates heat, melting the thermoplastic. In order to obtain a sealing the inductor is used together with a dolly.
The sealing is performed in such a way that the packaging material, which is to form a packaging, is fed stepwise or continuously, depending on the design of the apparatus. With the material lying immovable over the dolly a short while the inductor is applied against the material and is energized, so that the thermoplastic is melted. By a mechanical pressure between the inductor and the dolly with the thermoplastic molten a sealing is provided between two packaging material layers.
In accordance with previously known techniques, for example, as disclosed in applicant's U.S. Pat. No. 4,704,509, issued Nov. 3, 1987, (which patent is hereby incorporated by reference), conductor is for example constituted of a copper conductor cast into plastic material containing ferrite. As disclosed in applicant's above-identified U.S. Patent, the high frequencies normally employed in such sealing processes are normally between 2 MHz and 100 MHz.
A critical property of the inductor is the possibility to maintain the current conductor at a decently low working temperature in spite of a high-frequency on the sealing operations, i.e. to cool the current conductor. In a previously known inductor this is obtained in that the current conductor is a copper tube directly cooled with water. This design is very sensitive to mechanical loads, and the life span is short due to collapsed copper tubes/current conductors.
For this reason known conductors have worked against comparatively soft dollies, and their current conductors may have been provided with ridges for obtaining sufficient mechanical clamping force.
At sealing of new packaging materials, for example thermoplastic coated plate material, and filling of products, which are difficult to handle, for example highly viscous products, it is called for stronger inductors with improved possibilities to handle mechanical loads. These demands can hardly be fulfilled with inductors of conventional types.